The checks were related to a 2004 airworthiness directive, the FAA says

(CNN) -- United Airlines' fleet of 96 Boeing 757s was back in the air Wednesday after the airline grounded the planes for unscheduled maintenance a day earlier, a spokeswoman for the airline said.

The airline canceled 15 flights Tuesday because of the grounding, said Megan McCarthy, but flights were operating "100% normally" Wednesday with no cancellations.

United began follow-up maintenance checks on the affected planes' air data computers Tuesday, according to Rahsaan Johnson, another United spokesman.

"All of the air data computers are fully functional. The checks are necessary as part of a modification process to the system," Johnson said.

The Federal Aviation Administration said United's action involved its compliance with an airworthiness directive issued in 2004 that required a modification to the air data computer system to ensure that flight crews would be able to stop an overspeed or stall warning that sounded in error.